Function transform_async_context 

fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>)

Async desugaring uses an unsafe binder type ResumeTy to circumvert borrow-checking. The ResumeTy hides a &mut Context<'_> behind an unsafe raw pointer, and the get_context function is being used to convert that back to a &mut Context<'_>.

The actual should be &mut Context<'_>. This performs the substitution:

• create a new local _r of type ResumeTy;
• assign ResumeTy(transmute::<&mut Context<'_>, NonNull<Context<'_>>>(_2)) to that local;
• let all the code use _r instead of _2.

Ideally the async lowering would not use the ResumeTy/get_context indirection, but rather directly use &mut Context<'_>, however that would currently lead to higher-kinded lifetime errors. See https://github.com/rust-lang/rust/issues/105501.

The async lowering step and the type / lifetime inference / checking are still using the ResumeTy indirection for the time being, and that indirection is removed here. After this transform, the coroutine body only knows about &mut Context<'_>.